A group-III nitride semiconductor is a semiconductor using nitrogen as a group-V element in group III-V semiconductors. An aluminum nitride (AlN), a gallium nitride (GaN), and an indium nitride (InN) are typical examples of the group-III nitride semiconductor. In general, the group-III nitride semiconductor may be expressed as AlxInyGa1-x-yN (where 0≤x≤1, 0≤y≤1, and 0≤x+y≤1).
A high electron mobility transistor (HEMT) using such a nitride semiconductor has been proposed. The HEMT includes, for example, an electron transit layer made of GaN and an electron supply layer made of AlGaN epitaxially grown on the electron transit layer. A pair of source electrode and drain electrode is formed to be in contact with the electron supply layer, and a gate electrode is disposed therebetween. The gate electrode is disposed to face the electron supply layer with an insulating film disposed therebetween. Due to the polarization caused by the lattice mismatch of GaN and AlGaN, a two-dimensional (2D) electron gas is formed in an inward position by a few Å from an interface between the electron transit layer and the electron supply layer, within the electron transit layer. The source and the drain are connected through the 2D electron gas as a channel. When the 2D electron gas is blocked by applying a control voltage to the gate electrode, the source and the drain are disconnected from each other. In a state where the control voltage is not applied to the gate electrode, the source and the drain are conducted such that a normally ON type device is formed.
Since a device using a nitride semiconductor has the characteristics such as a high voltage, a high temperature operation, a large current density, high speed switching, and a low on-resistance, applications to power devices have been studied.
However, in order to be used as a power device, since it is required to be a normally OFF type device in which a current is cut off at the time of zero biasing, the aforementioned HEMT cannot be applied to the power device.
A structure for realizing a normally OFF type nitride semiconductor HEMT has been proposed. Specifically, an oxide film having a continuous interface from an interface between an electron supply layer and an electron transit layer is formed on the electron transit layer. Further, a gate electrode faces the electron transit layer with the oxide film interposed therebetween. In this configuration, since the electron supply layer is not present directly below the gate electrode, a 2D electron gas is not formed directly below the gate electrode. Thus, a normally OFF is achieved. The oxide film is created by, for example, thermally oxidizing a portion of the electron supply layer.
In a nitride semiconductor device having an oxide film directly below a gate electrode, an AlN layer may be used as an electron supply layer for the reason that the oxide film is easily created. However, when the AlN layer is used as the electron supply layer, the polarization caused by the lattice mismatch between the electron transit layer and the electron supply layer is so strong that a concentration of the 2D electron gas generated within the electron transit layer may be excessively high. The excessively high concentration of the 2D electron gas generated within the electron transit layer may cause an adverse effect such as a degradation of withstanding voltage of the nitride semiconductor device, an increase in a leakage current, an increase in capacity, etc. This problem may also arise in a case where a nitride semiconductor layer having a high Al composition is used as the electron supply layer, as well as in a case where the AlN layer is used as the electron supply layer.